The ruling by the U.S. Patent Trial and Appeal Board (PTAP) frees Bigoen Inc from future royalty payments to Forward and sent the Danish firm’s shares down nearly 20 percent. Biogen shares were up 1.4 percent.

The companies had been locked in a dispute over patents for dimethyl fumarate, the key active ingredient in Tecfidera, the leading oral MS treatment with sales of about $4 billion in 2016.

The PTAP ruling found Forward’s written patent description to be insufficient to declare infringement by Biogen.

Had Biogen lost it would have had to pay Forward a 10 percent royalty on Tecfidera sales starting in 2021.

Biogen’s Tecfidera exclusivity is secured through 2028 under the ruling.

Biogen in January had taken out something of an insurance policy against a potential defeat in the patent dispute. It agreed to pay Forward $1.25 billion to license the company’s patents covering multiple sclerosis drugs, giving Biogen a share of Forward’s intellectual property.